In a known implementation of such a network, the transmitters and receivers are linked to the input and output ports of the central switching node by means of dedicated electrical communications links, the node having sufficient switching power to be able to interconnect a desired number of ports.
Passive optical networks are emerging as a promising means of providing customers with broadband services, and are economically attractive for providing telephony and low data-rate services to customers requiring just a few lines. The telephony passive optical network (TPON) shares customer access costs by means of a passive splitting architecture to multiplex up to 128 customers, with current technologies, onto a single fibre at the exchange. With such a network in place, broadband services could easily be provided by the addition of more operating wavelengths. The first step towards a broadband passive optical network (EPON) would probably be to add just a few wavelengths, each allocated to a particular service such as broadcast TV, video library and ATN services, with each wavelength electronically multiplexed to provide sufficient numbers of channels. In the longer term, spectrally-controlled sources such as DFB lasers would allow extensive wavelength multiplexing, and the possibility of allocating wavelengths to individual customers or connections, to provide wavelength switching across the network.
It has therefore been proposed to link the sub-networks of transmitters and receivers to the central switching node by means of such passive optical networks, each transmitter of a sub-network transmitting information optically on an optical carrier of a fixed, distinct wavelength, and the various transmitted signals being passively multiplexed onto a single optical fibre for transmission to the central switching node. A demultiplexer at the central switching node would separate the signals according to wavelength, and convert each into an electrical signal. In this way, each transmitter is permanently linked to a distinct, input port of the central switching node. Similarly, the outgoing connections from the output ports of the central switching node to the optical receivers can be in the form of a passive optical network. The outgoing signal from each output port of the central switching node is converted to an optical signal of a wavelength corresponding to that which the receiver associated with that output port is configured to receive. These optical signals for the receivers of the sub-network are multiplexed onto a single outgoing optical fibre, which multiplex is passively split to each receiver. Each receiver selects the wavelength corresponding to it by the use, for example, of an optical filter or a coherent optical receiver. Such a network, employing passive optical sub-networks, requires a central node of the same switching power as that using dedicated electrical connections for the same interconnect power.
Another known interconnection arrangement uses wavelength switching or routing which is a simple but powerful technique for providing both one-to-one and broadcast connections between customers. One-to-one connections simply require each customer to have a tunable light source connected by a wavelength division multiplexer. Light can be directed from any transmitter to any receiver by tuning to an appropriate wavelength. A fast connection time of 2 nsec, has been demonstrated using a cleaved coupled cavity laser. Broadcast or distributed connections are naturally provided by a star coupler arrangement shown, for example, in patent specification EP-A-2,043,240. A star coupler splits the optical power from each input port to every output port so that, by using sources of fixed, distinct wavelengths, appropriate channels can be selected at the receivers by means of tunable optical filters. This has been demonstrated using an 8.times.8 array of wavelength-flattened fused=fibre couplers and position-tunable holographic filters recorded in dichromated gelatin, to tune across the entire long-wavelength window from 1250-1600 nm. An acousto-optic tunable filter has more recently been demonstrated with about 1 nm bandwidth, 260 nm tuning range, and a channel selection speed of 3 microseconds.
In common with optical space switches, use of wavelength switching in the local network would enable the full potential of optics to be realized, by providing a broadband optical switching and distribution capability, which is essentially transparent to the chosen signal bandwidth and modulation format. A large number of diverse optical technologies have been identified for achieving both space and wavelength switching in a local network environment.
However, both switching techniques have their limitations and disadvantages. For space switches, it is the sheer number, and hence cost, of crosspoints or equivalent switches needed to interconnect customers (from information theory the minimum growth rate that can be incurred is log.sub.2 (N!)). For wavelength switches, the problem is the limited number of available distinct wavelength channels, which restricts the number of customers, although this limitation can be overcome by employing wavelength switches in three or more stages of switching, which allows the same wavelengths to be re-used in different "switches". For space switches, the use of multi-stage networks can never overcome the log.sub.2 (N!) growth rate.